A block copolymer having a non-polar polymer block and a polar polymer block has features coming from both segments, and has a lot of possible applications. In order to give these properties, the microstructure of each segments has to fall within a certain range. A process to control the molecular structure of the resulting block copolymer as desired, with high yield and with good producibility, has been sought.
In general, a process for preparing such a block copolymer includes steps A and B as follows:
A) a step consisting of homopolymerizing or copolymerizing a conjugated diene monomer and/or an aromatic vinyl hydrocarbon monomer in order to obtain the non-polar polymer block; PA1 B) a step homopolymerizing or copolymerizing a polar monomer in order to obtain the polar polymer block.
And it is well known that a conjugated diene monomer, such as 1,3-butadiene, and isoprene, an aromatic vinyl hydrocarbon monomer, such as styrene, or a monomer containing a polar group (will be referred as a polar monomer), such as an ester of acrylic acid and methacrylic acid, and N,N-dialkylacrylamide can be polymerized by an anionic polymerization.
But to prepare a block copolymer containing a non-polar polymer block and a polar polymer block by an anionic polymerization is not straightforward.
In general, the carbanion at an active end of a polymer has a high reactivity towards a polar monomer and attacks not only vinyl linkage of a monomer but also other polar groups, such as a carbonyl group. As a result, there is a high possibility that polymerization would be disturbed. Further, because of these side reactions, to control the microstructure of the resulting polymer is difficult, which will lead to a formation of a polymer with undesirable properties.
To overcome these difficulties, a process which can lower the reactivity of the carbanion at the polymer end towards a polar monomer has been studied in order to provide a block copolymer containing both a non-polar polymer block and a polar polymer block.
Some of these processes include (1) to form the polar polymer block at a low temperature of below zero, (2) to add a 1,1-diphenylethylene compound to form a bulkier carbanion, which has lower reactivity towards a polar monomer, to react with the active ends of polymers before adding a polar monomer (D. Freiss, P. Rempp and H. Benoit, Polym. Lett., 2, 217, (1964)), and (3) to add a polar monomer in the presence of an excess amount of an ether compound, such as tetrahydrofuran (THF) (JP-A-1-131221).
Even though these processes of (1) to (3) can provide a block copolymer of desirable molecular structure and with desirable properties, in the process (1), since an anionic polymerization is often carried out at ambient temperature or higher temperature, cooling below zero causes a higher cost, viscosity increase, longer reaction time, and lower reactivity of the polar monomer; in the process (2), 1,1-diphenylethylene compound which has to be used is expensive and, sometimes, cooling is necessary; and in the process (3), the addition of an ether compound causes a declination in producibility, that is, if the ether compound is added before or during the formation of the non-polar polymer block, the content of vinyl linkage increases, which restricts the microstructure of the resulting polymer, and if the ether compound is added after the formation of the non-polar polymer block is completed, the impurities, such as water, in the ether compound deactivate some of the polymer ends, and will cause a formation of a polymer with only a non-polar polymer block and a loss of control over conversion of the polar monomer.